The present invention relates to a dielectric ceramic composition for a multilayer ceramic capacitor (abbreviated as MLC hereinafter), the MLC using the composition and a process of producing the same. Particularly, the present invention relates to a technology for modifying the composition of a dielectric ceramic composition to improve the thermal efficiency and the mass productivity of the process of producing a capacitor.
Following the demand for downsizing an MLC with higher performance, in recent years, MLC of small size and large capacitance have increasingly been required, so that MLC of more than 200 layers have been developed. These MLCs have conventionally been produced by the wet stack process by means of screen printer or the dry stack process of laminating green sheets together. According to these processes, organic binders, plasticizers and organic solvents are mixed into a dielectric powder, to prepare a paste or a slurry, which is then dried to prepare a dielectric sheet.
According to these processes, however, a larger volume of organic solvents should be used, which causes difficulty in the control of the evaporation of the solvents and induces non-negligible influence over humans and environment. Hence, these processes have been drawing social concern from an environmental standpoint. According to the dry stack process, the use of films of polypropylene (abbreviated as PP) and polyethylene terephthalate(abbreviated as PET) following the progress in the preparation of thin-film dielectric sheets of 5 microns or less causes an issue against the disposal of the films. According to the wet stack process by means of screen printer, alternatively, no PET film is used, therefore involving no issue about film disposal. Nevertheless, it has been difficult principally according to the process, to produce a uniform dielectric sheet of a thickness below 5 microns and without any pinhole.
So as to overcome these problems of the conventional processes, the present inventors have already proposed a novel process where aqueous binders can be used (see Japanese Patent Application No. 9-033759).
MLCs have been fired conventionally by aligning green chips on a ceramics setter of zirconia and magnesia, followed by binder burn out around 300.degree. C., elevating the temperature at a temperature rising rate of 50 to 200.degree. C./hour up to around 1,300.degree. C. by using a tunnel furnace and a batch furnace and retaining the temperature for several hours, and subsequently lowering the temperature at a temperature lowing rate of 50 to 200.degree. C/hour for firing.
So as to improve the environmental problems about global warming, meanwhile, a process at a high thermal efficiency has been desired in recent years. For example, a proposal has been made about a process of temperature rising or temperature falling at a rate of 600.degree. C. to 1,500.degree. C./hour for the improvement of the thermal efficiency during MLC production to complete firing for a shorter period of time. Various setter materials have been suggested for the achievement of such fast firing, including metal base firing setters comprising platinum-, nichrome-, molybdenum-, and tungsten-based metals and materials produced by coating ceramics of zirconia and magnesia on the setters.
When dielectric materials for general use (for example, Japanese Patent Laid-open No. 47-035751 and Japanese Patent Application No. 59-220745 and No. 60-008999) are used for firing at a temperature rising rate or a temperature falling rate of 600 to 1,500.degree. C./hour for a short period of time so as to improve the thermal efficiency for MLC production, disadvantageously, cracks readily emerge in the resulting elements after firing. The problem of the occurrence of cracking has been particularly noticeable during the production of large-type MLCs of a size of 5.0.times.5.7.times.2.0 mm and with an active dielectric layers of 5 microns or less.
For preparing a conventional barium titanate-based material satisfying the JIS B characteristic (a capasitance change at .+-.10% or less, -25.degree. C. to +85.degree. C.) into slurry during MLC production, additionally, the following problems have been remarked.
Regarding the aforementioned dielectric materials, first, binders of acrylic resins for general use readily cause slurry foaming and the occurrence of pinholes on green sheets, disadvantageously. When the resulting element is subjected to a wet barrel process for polishing the MLC corners, chipping also readily occurs, disadvantageously. These disadvantages are notable particularly when aqueous binders are used, which is a serious problem from the standpoint of mass production. Other than these disadvantages, additionally, a problem has been remarked, such that the breakdown voltage (BDV) is decreased below 200V, particularly when a large-size MLC of a 5.0.times.5.7.times.2.0 mm size and with a dielectric layer of 5 microns or less is to be produced.
The present invention has been proposed so as to overcome the problems of the prior art as described above. It is an object of the present invention to provide a dielectric ceramic composition preferable as a MLC, which can finely be slurried by using an aqueous binder and which can be sintered well with no occurrence of any crack therein even by fast firing and with rare occurrence of chipping therein after barrel treatment.
It is the other object of the present invention to provide a MLC capable of satisfying the JIS B characteristic including a high BDV even at a large size, greater performance and high reliability by using such composition, and to provide a method for producing a MLC at a high thermal efficiency and a great mass productivity and with sufficient environmental gentleness.